This invention relates to torque measuring apparatus employing strain gauge sensors for providing an output indicative of torque measured, and in particular, to electronic circuitry for processing the output of the strain gauge sensor and providing a digital display of the torque measured.
Electronic torque wrenches providing digital display of torque measured have been available for many years. In many designs, the torque transducer is mounted on a beam that is parallel to the handle of the wrench. Typically, the torque transducer comprises a pair of resistive strain gauges, physically mounted on opposite sides of the bending beam and electrically connected in a balanced bridge circuit. The strain gauges measure bending strain in the bending beam as torque is applied, providing an unbalance signal which is processed and used to generate a digital representation of the torque applied. Although these prior art torque wrenches provide increased accuracy through the use of electronic means for sensing and displaying applied torque, a significant drawback of such torque wrenches is their sensitivity to hand hold position. That is, the accuracy of the indication of the torque applied is dependent upon the position of the hands of the user on the torque wrench.
To minimize this problem, electronic torque wrenches were proposed in which the strain sensing elements were removed from the wrench handle. For example, in U.S. Pat. No. 4,125,016, there is disclosed an electronic torque wrench in which applied torque is measured by a torsion stud which is removably inserted in a socket structure near the end of the wrench handle such that torque is applied to the fastener or other work piece directly through the torsion stud. The body of the torsion stud is instrumented with strain-sensitive resistors to produce an analog signal representing applied torque. Although this construction eliminates the hand hold problem, the introduction of the removable torsion stud creates other problems. For example, such arrangement does not present a rigid mechanical structure because the element is held in place only by a set screw. With continued use, the set screw could work loose, releasing the torsion stud from its mount, allowing it to drop out of the torque wrench head. Also, the sensors carried by the torsion stud are connected to the electronic circuitry through electrical leads which extend through passageways in the torque wrench handle and the mount for the torsion stud in the head portion of the wrench. It would appear that such electrical connection to the sensors, caused by the removable torsion stud, could easily be broken.
In processing the electrical output of strain gauge sensors in electronic torque wrenches or other torque measuring apparatus, it is becoming common practice to provide peak hold operation to detect and temporarily display the peak value of the highest torque reached during a torquing operation. Various arrangements have been proposed for providing peak hold operation, the simplest of which include a capacitor which is charged by the gauge signal, that is, the output of the strain gauge. The output of the peak hold detector is applied to the digital display of the torque measuring apparatus. When the torque decreases from its peak value, there is a corresponding decrease in the gauge signal. A unidirectional circuit element connected in the capacitor charging circuit becomes reverse biased when the gauge signal decreases, enabling the capacitor to temporarily hold its charge upon decrease of the gauge signal, so that the output of the peak hold circuit as applied to the display, causes the display to temporarily maintain an indication of the highest torque reached. In more sophisticated torque measuring systems, in which torque signals are processed by microprocessor, the torque values sensed are provided by a sensor temporarily stored in memory and the memory content is updated periodically under microprocessor control. Peak detection can be made by comparing successive values of torque sensed. Whether the torque signals are processed directly by analog to digital means or by microprocessor means, the conversion is necessarily done in some period of time. If there is a change in the torque during the update period, this can result in a large error due to the change in the torque signal input during the conversion cycle. The amount of error is dependent upon the duration of the update period, but can approximate ten to twelve percent for an update period in the order of 300 milliseconds.